Possibly a shortening of Chunder Loo,rhyming slang for spew (said to be derived from the cartoon character “Chunder Loo of Akim Foo”, drawn by Norman Lindsay for a series of boot-polish advertisements in the early 1900s), but the rhyming slang usage is not recorded. The derivation from a supposed nautical phrase Watch under!, used to warn people on lower decks that someone above was vomiting over the side of the ship, is wholly unsubstantiated.

2001 September 9, John Dean, “‘chunder’”, in alt.usage.english, Usenet[2]:

I would guess it points up the difference between the involuntary chunder where you cannot choose the time place or direction, and the self-induced chunder which facilitates further consumption of alcohol after your theoretical limit is reached.

There are plenty of winding roads, diesel fumes, crowded public transport and various less than sweet odours to get you chundering when you′re on the move in this part of the world, so take a good supply of motion sickness remedies if you know you′re susceptible to this.